Before Jupiter settled into its current orbit, the massive planet came in like a wrecking ball, sweeping its way through the early solar system and smashing a first generation of inner planets. Earth and the other rocky planets later coalesced out of the debris, according to findings published in Proceedings of the National Academy of Sciences this week.
The structure of our solar system is unlike hundreds of other planetary systems: While known extrasolar systems have large planets that orbit close to their host star, we’ve got low-mass inner planets that are far from the sun. "One of the most interesting features is the absence of planets inside the orbit of Mercury," Greg Laughlin from the University of California, Santa Cruz, says in a news release. "The standard issue planetary system in our galaxy seems to be a set of super-Earths with alarmingly short orbital periods. Our solar system is looking increasingly like an oddball."
To see why we’re such an unusual member of the galactic planetary census, Laughlin and Caltech’s Konstantin Batygin modeled planetary formation in the early solar system. Jupiter migrated inwards, they found, from an orbit of more than 5 astronomical units (AUs) to around 1.5 AUs (where Mars' orbit is today). This was then followed by an outward orbital migration to its current position—triggered by the formation of Saturn.
This inward-then-outward wandering may explain the gaping hole in our inner solar system, as well as the low mass of inner planets. It’s possible that prior to Jupiter’s inward migration, planets forming close to the sun were on their way to becoming super-Earths—planets more massive than Earth but smaller than Neptune. According to this simulation, gravity from Jupiter’s inward migration shepherded these small planetary precursors (along with asteroids and other bodies) into close-knit, overlapping elliptical orbits.
This would have initiated a cascade of collisions in the inner solar system, and any preexisting planets and planetesimals near the sun would have been smashed into pieces and sent hurling towards a fiery death. "It's a very effective physical process," Batygin says in a university statement. "You only need a few Earth masses worth of material to drive tens of Earth masses worth of planets into the sun."
The remaining mass-depleted debris of planet-making material later formed into the second generation inner rocky planets: Mercury, Venus, Earth, and Mars. That’s consistent with evidence that these smaller, terrestrial planets with thinner atmospheres are younger than the outer planets.
Based on these calculations, stars that harbor giant planets with orbital periods longer than 100 days are unlikely to host multiple close-in planets. In the diagram to the right, you can see how most extrasolar planets smaller than Jupiter are much closer to their host stars than our innermost planets are to the sun.
Furthermore, by the time Earth formed, the disk of hydrogen and helium gas was long gone. Most super-Earths orbiting other stars have a substantial hydrogen atmosphere since they formed when the gas was still abundant in their planetary disk. "Ultimately, what this means is that planets truly like Earth are intrinsically not very common," Batygin adds.
Images: K.Batygin/Caltech (top), Batygin and Laughlin, 2015 PNAS (middle)